The present invention relates to a process for forming fine-celled foamed fibers, and especially to such processes employing a combination of molten polymer containing therein, a dissolved decomposable compound or gaseous blowing agent, extrusion (spinning) conditions and elements of the extrusion (spinning) apparatus. The present invention also includes novel foamed polyamide fibers which may, but are not necessarily, produced by the present process.
Foamed thermoplastic (and especially polyamide) fibers have been produced, especially for the purpose of being broken (fibrillated) into three-dimensional structures of interconnected fiber elements. See U.K. Patent Specification Nos. 1,221,488 (1971), 1,296,710 (1972) 1,316,465 (1973) and 1,318,964 (1973), all of Monsanto Chemicals Limited. Water is said to be the preferred blowing agent in most of the cases, and nucleating agents such as talc or powdered silica are also used.
Foaming polyester and polyamide fibers for textile applications is disclosed in DOS No. 2,148,588 (Apr. 5, 1973) (see Example 7). See also Chem. Abstr. 90:24692m (1979) of Japanese Kokai No. 78,106,770.
Screens or porous discs are conventionally used in fiber spinning to help to distribute the polymer or to create sheer or pressure drop or to remove grit in various solid particles from the polymer melt. The latter effect is to prevent such solid particles from clogging the spinning orifices (capillaries). See, for example. U.S. Pat. Nos. 3,006,028 to Calhoun (1961), 3,028,627 to McCormick (1962), 3,295,161 to Mott (1967), and 3,847,524 to Mott (1974). Applicants are not aware of any specific teaching to use fine screen structures in spinning polymer containing a blowing agent.
Hollow fibers are also known to the art containing elongated voids (usually extending long distances or the entire length of each filament in the longitudinal direction). Such fibers contain large void volumes and find use in thermal insulation. The voids are generally produced by the use of modified spinning dies.